The field of the invention is cables for use in environments exposed to moisture which nonetheless contain telecommunication elements which must be protected against moisture.
Longitudinally extending telecommunication elements in telecommunication cables include both electrical conductors and optical waveguides. It is well known that both such telecommunication elements must be protected against contact with moisture. Moisture can cause electrical shorts in electrical conductors and can cause bending in optical waveguide fibers, especially when water freezes, resulting in increased attenuation of such optical fibers.
Moisture can enter a cable due to differences in vapor pressure between a cable interior and its external environment and condense within the interior due to temperature cycling.
Water may enter a cable due to damage to the cable outer jacket due to rodent activity or other mechanical breaks or team. Upon entry, water may then migrate longitudinally along the cable.
Various methods have been used to protect telecommunications cables from the deleterious effects of moisture when moisture enters the cable or spreads longitudinally through the cable. Various oil or grease based compounds have been used to block the spread of moisture in the cables. However, a disadvantage of such compounds is that they are rather messy when workers must enter or reenter a cable to split off or connectorize individual telecommunication elements in a multiconductor cable.
By way of example, U.S. Pat. No. 4,512,827 teaches an electric cable which includes powdered electrical insulating material mixed with a small quantity of liquid hydrophobic material.
Another method of protecting a cable from moisture is the use of a sealed metallic shield inside the cable jacket. Disadvantages of such shields are various problems encountered in completely sealing the seams, the low processing line speeds necessary to form and seal such seams, and the introduction of metallic elements into what otherwise may well be an all-dielectric cable.
Another method used to protect telecommunication cables from the effects of moisture is to introduce into the cables powders which expand to many times their initial size upon contact with water, such powders sometimes called superabsorbent powders. Two such superabsorbent powder materials are cellulosic or starch-graft copolymers and synthetic materials. Synthetic materials include polyelectrolytes and nonelectrolytes. Polyelectrolyte superabsorbent materials include polymaleic anhydride-vinyl, polyacrylonitrile-based materials, polyacrylic acids, and polyvinyl alcohols. The polyacrilic acids include homopolymers and copolymers of acrylate esters and acrylic acids.
A disadvantage of the use of such water-absorptive powders is that the powder can fall out of the cable during processing or cable reentry. Therefore, various means have been used to bind the powders to cable materials, including the use of adhesives or electrostatic fields to bind the powders to other cable elements. However, the adhesives reduce the surface area of the powder available to absorb moisture, and the electrostatic fields can dissipate over time.
Another way to introduce a superabsorbent powder into a cable is to include the powder on or in between layers of a tape or yarn which is either helically or longitudinally wrapped around the cable core. A disadvantage of the use of such tapes or yarns is their expense. Another is the space such tapes or yarns can take up within a cable, thereby increasing the cable outer diameter.
U.S. Pat. Nos. 4,913,517 and 5,389,442 disclose cables including fibrous strength members, made for example of aramid yarns such as Kevlar.RTM. yarns, which have been pretreated before cabling by being impregnated with a superabsorbent material derived from an aqueous solution. The aqueous solution comprises acrylate polymeric material including acrylic acid and sodium acrylate functionalities and water. However, the strength yarns must be impregnated and dried to provide a film in and around the interstices of the fibrous strength members, and later cabled in a separate processing step.